The use of chromogenic, that is dye-labelled, substrates for the assay of polysaccharide endo-hydrolases is well known; however the substrates used hitherto suffer from serious disadvantages. Thus most substrates currently available are insoluble, for example those marketed under the trade marks "Phadebas" (Pharmacia South Seas Co.) and "Amylochrome" (Roche Diagnostics), and also dyed amylose (Calbiochem Co.) and amylopectin (Sigma Chemical Co.).
Two soluble dyed substrates are available, namely "Dyamyl-L" (Reactone Red 2B-amylopectin) from General Diagnostics and Lyosine Red (P.. Brilliant Red M-2B5-Amylopectin) from Reliable Reagents Co. Soluble substrates are advantageous because in general they are more susceptible to enzyme hydrolysis, thereby imparting a greater sensitivity to the assay procedure; also enzymes have a greater affinity for soluble substrates so that in order to obtain maximum reaction rates much lower concentrations of the substrate can be used in the assay mixture. However, the soluble substrates currently available are difficult to prepare and are correspondingly expensive; one difficulty in their preparation is the separation of the dyed polysaccharide from free dye which involves a laborious and time consuming gel filtration step using a media such as the molecular sieve "Sephadex G25".